Ghost notes are when you hold down a note without sounding it by either plucking it or hammering it. You'll get a slight sound, but it's very light. Blues players use them alot usually before sliding to another fret, -(12)/15- or before bending -(12)b13r12. It's hard to understand them with just tab, because they do look like tie notes and can be confused. You usually need the actually written music to understand how to play them better. With written music they actually give you a note duration so you how how long to hold the ghost note before moving on to the next note. They take alittle discipline to get used too, because instinctively you'll want to either pluck the string or hammer too hard.